This invention relates generally to the field of data storage devices and more particularly but without limitation to reducing resonant oscillation of the data storage device mechanical structure.
Modern data storage devices such as disc drives are commonly used in a multitude of computer environments to store large amounts of data in a form that is readily available to a user. Generally, a disc drive has a magnetic disc, or two or more stacked magnetic discs, that are rotated by a motor at high speeds. Each disc has a data storage surface divided into a series of generally concentric data tracks where data is stored in the form of magnetic flux transitions.
A data transfer member (sometimes referred to as a read/write head) such as a magnetic transducer is moved by an actuator assembly to selected positions adjacent the data storage. The active elements of the read/write head are supported by suspension structures extending from the actuator assembly. The active elements are maintained a small distance above the data storage surface as the read/write head flies upon an air bearing generated by air currents caused by the spinning discs.
Each read/write head is typically provided with separate read and write elements, with a common configuration utilizing a thin film, inductive write element and a magneto-resistive (MR) read element. Data are written by passing a write current through the write element, with the write current generating a time-varying magnetic field which accordingly magnetizes the disc surface. Previously written data are read using the read element to transduce the selective magnetization of the disc to generate a read signal which is received by a read channel to reconstruct the data.
The actuator assembly operates within a negative feedback, closed-loop servo system. In this manner, the actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations. A servo controller samples the position of the read/write heads relative to some reference point and generates an error signal based upon the difference between the actual position and the reference position. This error signal is then used to drive the data head to the desired reference point, typically by demanding a current through a voice coil motor (VCM) which forms a part of the actuator assembly.
Thus, a disc drive mechanical structure is composed of multiple mechanical components that are pieced together to form the final disc drive assembly. Each of these components has various resonant modes that if excited by an external energy source will cause the part to physically move at the natural frequencies of oscillation for the component in question. This movement can occur in a bending mode, a twisting mode or a combination of the two. If the component is highly undamped (i.e. the resonance is high amplitude, narrow frequency band) it will tend to oscillate with a minimal external driving energy. This oscillation results in physical motion of the read/write head, causing off track errors and potential fly height problems. These oscillations are often referred to as xe2x80x9cresonances.xe2x80x9d
If resonances occur in a disc drive, they can severely limit drive performance, both in seek mode and track-follow mode. To obtain the optimal disc drive performance requires that there be no resonances present. However, this scenario is not physically possible. Every mechanical component has a natural frequency of oscillation. Nevertheless, it is desirable to reduce or minimize the resonances. One way of doing this is to mechanically damp the mechanical components and thereby decrease the amplitude of the resonant mode. This can be done by careful design, the end result being a reduction in the amplitude of the oscillation to a level that is deemed acceptable to achieve a desired drive performance.
However, there are situations where a component is not able to be mechanically damped. This could occur, for example, because of materials used or because of design time constraints. When this scenario occurs, the only way to improve drive performance is to make sure that no excitation energy at the natural frequency of oscillation reaches the mechanical component to start it oscillating. The present invention concentrates on this approach.
As mentioned above, typical disc drives demand a current through a voice coil motor (VCM) to drive the read/write head to the desired position. When a frequency spectrum of demand current is analyzed it is found that the spectrum is composed of frequency components from direct current (DC) all the way up to multiple kilohertz (KHz). If VCM current is driving the actuator assembly at the same frequency as the natural frequency of a mechanical resonant mode of a mechanical component, the energy may be sufficient to excite the mechanical structure into oscillation. This is very undesirable and will at least degrade disc drive performance or at worst will cause the servo system to go unstable.
The method employed by servo engineers to minimize the chances of the mechanics oscillating is to use hardware electronic filtering and/or digital filtering of the VCM current via a microprocessor or digital signal processor. Both types of filters achieve the same overall result in that they reduce the driving force energy (i.e. the current flowing) at frequencies deemed a concern.
One type of filter that is widely used to remove driving energy at the mechanical resonant modes is known as a notch filter. A notch filter is a band-rejection filter that produces a sharp notch in the frequency response curve of the disc drive. When a notch filter is activated by the servo control loop, the open loop response ends up a summation of the original response plus the notch filter response. If the notch filter is centered about the frequency where the peak amplitude of the mechanical resonance occurs, then the driving force energy at this frequency can be reduced so that there will be little or no energy made available to excite the mechanical structure.
One problem associated with notch filters, however, is that if the center frequency of the mechanical resonance does not align with the center frequency of the notch filter then the attenuation of the driving current may not be sufficient to prevent the structure from oscillating. This surely occurs when the mechanical resonance shifts in frequency, and often occurs due to the part-to-part variation between individual disc drives.
One solution is to include a number of notch filters designed to cover a spread in mechanics. Such a filter, for example, is described in U.S. Pat. No. 5,032,776. These filters remove some driving energy unnecessarily, however, such as at non-resonant frequencies. Such approaches do not provide the optimal solutions, and do not guarantee that resonance won""t occur.
Another solution is to calculate and store a notch filter for each of the heads in an actuator assembly, such as is described in U.S. Pat. No. 6,246,536. These and other similar approaches that attempt to reduce the mechanical complexity of the structure do not focus on deriving an optimal filter response. Rather, the filter sought after is one that simplistically computes one or more digital notch filters associated with the peak resonances in the structure frequency response. It has been determined that an optimal composite attenuating filter is derived by first summing the observed frequency response of the structure with the frequency response of the first computed notch filter to derive a modified frequency response of the structure, then computing the next notch filter on the basis of the modified frequency response of the structure. Such an approach empirically aligns the attenuating frequencies of the composite attenuating filter with the resonant frequencies of the structure, thereby minimizing the driving energy necessary to prevent oscillation. It is to these improvements and others as exemplified by the description and appended claims that embodiments of the present invention are directed.
Embodiments of the present invention are directed to a data storage device comprising a data storage disc adapted to store data and an actuator assembly. The actuator assembly comprises a read/write head adapted to read data from and write data to the disc, and an actuator arm coupled to the head and controllably positionable to move the head relative to the disc in response to a driving energy. The data storage device further comprises a servo control circuit providing the driving energy, comprising an attenuating filter limiting the driving energy at resonant frequencies of the data storage device mechanical structure, the attenuating filter constructed by a process comprising: (a) initiating a track seek condition moving the head to a selected track of the data storage disc; (b) measuring the structure frequency response of the data storage device in terms of magnitude versus frequency between selected first and second frequencies; (c) determining the peak amplitude of the magnitude in step (b); (d) determining the frequency associated with the maximum amplitude of step (c); (e) computing a notch filter centered at the frequency of step (d); (f) saving the notch filter in memory; (g) creating a theoretical frequency domain of the attenuating filter as the sum of all the notch filters in memory of step (f) in terms of magnitude versus frequency; (h) combining the frequency response of the structure from step (b) and the frequency domain of the attenuating filter from step (g), deriving a modified structure frequency response; (i) substituting the modified structure frequency response of step (h) for the structure frequency response in step (b) and repeating steps (c) through (h) until the peak amplitude of step (c) is less than a desired magnitude; and (j) combining all the notch filters in memory of step (f), defining the attenuating filter; and a servo control processor recalling and implementing the attenuating filter of step (j), controlling the driving energy to position the actuator assembly.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.